The invention relates to a method for automatic drift stabilization in radiation measurement with a detector, the pulse amplitude integral of which measurement is a standard for the radiation energy absorbed by the detector, using control devices connected to the output side of the detector, in which a defined threshold in the pulse amplitude spectrum is used to adjust the amplification in such a manner that the rate of all the pulses having a pulse height greater in comparison with a threshold is kept constant, and the measurement outcome is correspondingly corrected thereby.
A method of this generic type is disclosed in German Patent 18 09 520, which issued on Oct. 11, 1979. In that method, a threshold S of a pulse magnitude G is defined such that the number, or rate, of pulses with a magnitude above this threshold value S can be "split off" with the aid of an integral discriminator, and this pulse rate can be kept constant.
This means that changes in the number of these pulses, which are for instance dictated by undesirable changes in amplification in the downstream evaluation circuit, can be reliably eliminated; that is, the control has a drift-stabilizing effect. Points in the pulse spectrum that are located on the edge of a peak are especially suitable as an "orientation point" for such a threshold S, because in that case a particularly sensitive drift stabilization can be carried out.
A special feature of the method described in the above-cited patent is that the measurement radiation source itself, or its pulse height distribution, is used for drift stabilization.
Stabilization with an additional external light source is disclosed in Swiss Patent 665 291, which issued on Apr. 29, 1988. Here, a scintillation detector is supplied not only by the radiation source used for the actual measurement but also by a further light source, in a modulated form, so that the downstream photoelectric converter can make a distinction from the light pulses originating in the irradiation radiation source. The number of reference pulses from the source is then used to stabilize the scintillation detector.
Each of these methods has specific disadvantages. Since the method of German Patent 18 09 520 also uses the measurement radiation source as a reference source for the drift stabilization, measurement radiation of sufficient intensity must always reach the detector.
Yet precisely this does not occur, for instance with continuous fill level measurements. The measurement radiation intensity at the detector fluctuates markedly with the fill level, and as a rule is absorbed completely when the container is full.
In radiometric fill level measurements, not only NaI scintillators and a rod irradiator but also large-volume plastic scintillators in rod form with lengths of up to 2 m, and spot irradiators, are used. While the measurement accuracy for fill level measurements does not require drift stabilization in the case of NaI detectors, drift stabilization is necessary in large-volume plastic scintillators, because of the lower yield of fluorescence and poorer collection of light.
The desire for using large-volume plastic scintillators may have led to the solution described in Swiss Patent 665 291 with the additional external reference light source. However, in terms of apparatus, this embodiment is quite complicated because of the necessary additional electronic components.